Use abspath to dereference $BUILD_BASE

If the user tries to change BUILD_BASE to put the build products outside
the build tree the compile will fail due to hard coded assumptions that
$BUILD_BASE is a relative path. Fix by using $(abspath $(BUILD_BASE))
to rationalize to an absolute path every time and remove the relative
path assumptions.

This patch also adds documentation that BUILD_BASE can be specified by
the user.

Signed-off-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib1af874de658484aaffc672f30029b852d2489c8
diff --git a/Makefile b/Makefile
index fa711e2..6e14bdc 100644
--- a/Makefile
+++ b/Makefile
@@ -451,8 +451,10 @@
 
 include ${MAKE_HELPERS_DIRECTORY}plat_helpers.mk
 
-BUILD_BASE		:=	./build
-BUILD_PLAT		:=	${BUILD_BASE}/${PLAT}/${BUILD_TYPE}
+ifeq (${BUILD_BASE},)
+     BUILD_BASE		:=	./build
+endif
+BUILD_PLAT		:=	$(abspath ${BUILD_BASE})/${PLAT}/${BUILD_TYPE}
 
 SPDS			:=	$(sort $(filter-out none, $(patsubst services/spd/%,%,$(wildcard services/spd/*))))